apps_930
There is a building with n rooms, numbered 1 to n. We can move from any room to any other room in the building. Let us call the following event a move: a person in some room i goes to another room j~ (i \neq j). Initially, there was one person in each room in the building. After that, we know that there were exactly k moves happened up to now. We are interested in the number of people in each of the n rooms now. How many combinations of numbers of people in the n rooms are possible? Find the count modulo (10^9 + 7).
-----Constraints-----
- All values in input are integers.
- 3 \leq n \leq 2 \times 10^5
- 2 \leq k \leq 10^9
-----Input----- Input is given from Standard Input in the following format: n k
-----Output----- Print the number of possible combinations of numbers of people in the n rooms now, modulo (10^9 + 7).
-----Sample Input----- 3 2
-----Sample Output----- 10
Let c_1, c_2, and c_3 be the number of people in Room 1, 2, and 3 now, respectively. There are 10 possible combination of (c_1, c_2, c_3):
- (0, 0, 3)
- (0, 1, 2)
- (0, 2, 1)
- (0, 3, 0)
- (1, 0, 2)
- (1, 1, 1)
- (1, 2, 0)
- (2, 0, 1)
- (2, 1, 0)
- (3, 0, 0) For example, (c_1, c_2, c_3) will be (0, 1, 2) if the person in Room 1 goes to Room 2 and then one of the persons in Room 2 goes to Room 3.
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